The Light Switch Room

Problem

You are standing outside a closed room. On the wall beside you are three light switches, each connected to one of three standard incandescent light bulbs inside the room. All switches are currently in the off position, and the door is closed so you cannot see inside.

You may flip any of the three switches as many times as you like. However, once you open the door and step into the room, you may not return to the switches. You get exactly one trip into the room.

After entering, you must correctly identify which switch controls which bulb. How do you do it?